Excessive weighing is damagaging. there is no point in weighing your child a child twice a week. I only weigh my son once every three months, I also measure his height.
I think you get a better idea looking at the label on their clothes. If your three year old needs seven to eight year old clothes, it doesn't take a consultant paediatrian to know you got a problem.
My son is seven years old in a months time and is wearing seven to eight year old clothes. He is still overweight according to the charts, but I don't think it is desperate anymore. Infact he swam 75 metres today and is very active. I know that he will only get fitter.